30 THEORIES OF SPONTANEOUS GENERATION 



Stopped and the flask was allowed to cool. Under this treat- 

 ment the contents of the flask remained unchanged although, 

 in this case, the solution was directly connected through the 

 curved neck with the surrounding atmosphere. This was due 

 to the fact that all particles of dust, including the germs of 

 the micro-organisms, were retained on the curved surfaces 

 of the S-shaped tube. If the neck was cut off the liquid was 

 soon colonised by micro-organisms. In this experiment the 

 air was submitted to absolutely no treatment and neverthe- 

 less decomposition of the liquid did not occur, simply because 

 the organisms floating in the air were denied access to it. 



Further investigations by Pasteur showed that the content 

 of viable micro-organisms in the air was far from constant 

 and changed according to conditions such as season and place. 

 The largest number of germs is present in the air of towns 

 and inhabited places. The air of fields and forests is less rich 

 in micro-organisms, and finally in the mountains, especially 

 at great heights, the number of these minute living creatures 

 floating in the air is quite insignificant. One may therefore 

 open flasks containing sterile liquids without their necessarily 

 being exposed to infection. In many cases such flasks 

 remained sterile after resealing, although untreated mountain 

 air had been admitted to them. 



Pasteur also demonstrated that the air is far from being 

 the only source of infection of organic liquids. The germs 

 of micro-organisms are present on the surfaces of all the 

 objects which we use in the course of an experiment. There- 

 fore all these objects must be meticulously disinfected. 

 Pasteur showed that the appearance of micro-organisms in 

 the experiments of earlier investigators was always due to the 

 fact that they had not carefully eliminated all sources of 

 infection. Thus, for example, Pasteur showed by direct 

 experiments that the source of infection of Gay-Lussac's grape 

 juice was micro-organisms present on the surface of the 

 mercury. In other cases the organisms were derived from 

 incompletely sterilised utensils. If all sources of error are 

 avoided then, as Pasteur demonstrated brilliantly in numer- 

 ous experiments, infection will be absent in a hundred per 

 cent of cases. Pasteur also succeeded in showing that it is 

 possible to keep even such easily decomposed liquids as urine 



